Method and system for changing forward traffic channel power allocation during soft handoff

ABSTRACT

A system and method for adjusting forward traffic channel power allocation in a communications system, wherein signal qualities of pilot channels respectively transmitted by multiple base stations in an active set of a mobile station, are measured by the mobile station, compared to a signal quality standard, and the comparison results reported to a system controller, thereby to indicate which of the pilots at the mobile station surpass the standard. The system controller then adjusts the forward channel power allocation based on the comparison results.

[0001] This application is a continuation of U.S. patent applicationSer. No. 08/925,518, filed Dec. 27, 2000, entitled Method and Apparatusfor Changing Forward Traffic Channel Power Allocation, which is aContinued Prosecution Application of U.S. patent application Ser. No.08/925,518, filed Sep. 8, 1997 of the same title, both assigned to theassignee of the present invention

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to cellular communications systemsand more particularly to methods and an apparatus for changing forwardtraffic channel power allocation in a code division multiple access(CDMA) cellular communications system.

[0004] II. Discussion of the Background

[0005] In a CDMA cellular telecommunications system, a common frequencyband is typically used for communicating from a mobile to a set of basestations, and another common frequency band is typically used tocommunicate to the mobile from the set of base stations. In otherinstances, a common set of frequency bands may be used to conductcommunications. A primary benefit of transmitting multiplecommunications over a common frequency band is an increase in thecapacity of the cellular telephone system. The IS-95 standard,promulgated by the Telecommunications Industry Association (TIA), is anexample of a highly efficient CDMA over-the-air interface that can beused for implementing a cellular telephone system.

[0006] The set of communications conducted over the same bandwidth in aCDMA cellular telecommunications systems are separated and distinguishedfrom one another by modulating and demodulating the data transmittedusing pseudo-random noise (PN) codes known to both the receive andtransmit systems. The other communications appear as background noiseduring the processing of any particular communication. Because the othercommunications appear as background noise, CDMA protocols such as IS-95often employ extensive transmit power control in order to use theavailable bandwidth more efficiently. The transmit power control keepsthe transmit power of each communication near the minimum necessary inorder to conduct communications successfully. Such transmit powercontrol facilitates the processing of any particular communication byreducing the level of background noise generated by the othercommunications.

[0007] Another benefit of having base stations transmit to mobiles onthe same frequency band, and of having mobiles transmit to base stationon a second frequency band, is that “soft handoff” may be used totransition a mobile from the coverage area of a first base station tothe coverage area of a second base station. Soft handoff is the processof simultaneously interfacing a mobile with two or more base stations.Soft handoff can be contrasted with hard handoff during which theinterface with the first base station is terminated before the interfacewith the second base station is established.

[0008] As one might expect, soft handoff is generally more robust thanhard handoff because as least one connection is maintained at all times.Methods and systems for conducting soft handoff in a CDMA cellulartelephone system are disclosed in U.S. Pat. No. 5,101,501, filed Nov. 7,1989, entitled “Method and System for Providing a Soft Handoff andCommunications in a CDMA Cellular Telephone System”, and U.S. Pat. No.5,267,261, entitled “Mobile Station Assisted Soft Handoff in a CDMACellular Communication System”, both assigned to the assignee of thepresent invention and incorporated herein by reference.

[0009] In accordance with the soft handoff procedure described in theabove referenced patents, each base station transmits a respective pilotchannel that is used by the mobiles to obtain initial systemsynchronization and provide robust time, frequency and phase tracking ofthe cell-site transmitted signals. The pilot channel transmitted by eachbase station uses a common spreading code (i.e., pseudo-noise sequence)but uses a different code phase offset so that the mobile candistinguish the pilot channels transmitted from the respective basestations.

[0010] During a soft handoff, two or more base stations transmit thesame forward link data to the mobile. The mobile receives the signalsfrom the set of base stations and combines them. A method and apparatusfor performing such combining is described in U.S. Pat. No. 5,109,390,filed Nov. 7, 1989, entitled “Diversity Receiver in a CDMA CellularTelephone System”, assigned to the assignee of the present invention andincorporated herein by reference, discloses a diversity combining methodfor use in a CDMA cellular telephone system.

[0011] While soft handoff provides a more robust connection, in someinstances soft handoff also has a negative effect on the overallcapacity of the CDMA cellular telephone system. This is because themultiple forward link transmissions generated during a soft handoff mayincrease the total transmit power used to conduct the correspondingcommunication. This increased transmit power increases the totalbackground noise generated by the system, which in turn may decreaseoverall system capacity.

[0012] Whether soft handoff increases or decreases system capacity istypically dependent on the environment the mobile is experiencing duringsoft handoff. If the mobile is experiencing a fading environment, theincreased diversity provided by soft handoff is generally beneficial tosystem performance because the signals generally fade independently.When the mobile is in a non-fading environment, however, the diversityof data source is typically redundant. Therefore, for non-fadingenvironments the benefit provided by increased diversity of signalsource typically does not justify the overall increase in transmit powercaused by soft handoff.

[0013] Thus, the present invention is directed to improving theperformance of a CDMA telecommunications system by optimizing theconfiguration of a CDMA communications system during a soft handoff, ina multicarrier environment, or both in response to the environment inwhich the communications are being conducted.

SUMMARY OF THE INVENTION

[0014] Accordingly, one object of this invention is to provide a novelmethod for reducing a total amount of forward traffic channel powertransmitted to a mobile during soft handoff.

[0015] Another object of the invention is to provide a system thatimplements the aforementioned method.

[0016] Another object of the invention is to determine the environmentin which the mobile is operating during soft handoff, and optimize theconfiguration of the soft handoff in response to that determination.

[0017] The invention is equally applicable to a multi-carrier forwardlink.

[0018] Accordingly, one object of this invention is to provide a novelmethod for reducing a total amount of forward traffic channel powertransmitted to a mobile with a multi-carrier forward link.

[0019] Another object of the invention is to provide a system thatimplements the aforementioned method.

[0020] Another object of the invention is to determine the environmentin which the mobile is operating, and optimize the configuration of themulti-carrier forward link in response to that determination.

[0021] The present invention is applicable to systems which employ bothsoft handoff and a multi-carrier forward link.

[0022] The present invention provides a novel method and system where amobile frequently sends a bit-vector message to a system controllerindicating quantified, measured signal qualities (e.g., signal tointerference ratios) of pilots from each base station in an “active set”of pilot channels tracked by the mobile. The mobile generates thebit-vector message by monitoring the respective signal qualities of thepilots, comparing the respective pilot channel qualities against astandard, and transmitting the bit-vector message to the respective basestations in the mobile's active set, which then forwards the informationin the bit-vector message to a system controller. In response, thesystem controller issues a command to the base stations in the mobile'sactive set, adjusting selected ones of the respective code channelpowers of the base stations in accordance with the respective pilotchannel qualities reported in the bit-vector message generated by themobile.

[0023] Because the forward traffic channel includes the respective codechannels of the base stations in the mobile's active set, reducing thetransmit powers of the respective code channels reduces the transmittedpower of the forward traffic channel. Accordingly, the total capacity ofthe CDMA communication system increases as a result of radiating theminimum required forward traffic channel power necessary for properreception at the mobile. By rapidly communicating to the systemcontroller the observed pilot channel qualities, the CDMA system iscapable of rapidly re-optimizing system resources in response toenvironmental changes to maximize system communications capacity.

[0024] In an alternative embodiment of the invention which employs amulticarrier link, the mobile station sends a bit for every carrier, oralternatively a bit for every antenna. Additionally, the base stationadjusts the power on each carrier independently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

[0026]FIG. 1 is a block diagram of an exemplary CDMA cellular telephonesystem in accordance with the present invention;

[0027]FIG. 2 is a graph of pilot channel quality versus time and a softhandoff region displayed on the graph;

[0028]FIG. 3 is a block diagram of a mobile;

[0029]FIG. 4 is a graph showing exemplary probability of frame errorrate vs. Eb/No for various numbers of transmitting base stations asreceived by an N-finger diversity receiver;

[0030]FIG. 5A is a graph showing Ec/Io vs. time within a soft-handoffregion for three exemplary pilots;

[0031]FIG. 5B is a graph similar to that shown in FIG. 5a with theaddition of a threshold signal Δr which is formed below a highest pilotlevel;

[0032]FIG. 6A is a diagram of a first data structure for the bit-vectormessage indicative of pilot channel quality;

[0033]FIG. 6B is a diagram of a second data structure for the bit-vectormessage indicative of pilot channel quality;

[0034]FIG. 6C is a diagram of a third data structure for the bit-vectormessage indicative of pilot channel quality;

[0035]FIG. 7 is a flow diagram of a message sequence for reducing atotal amount of forward traffic channel power transmitted from basestations in an active set when excess power is being transmitted;

[0036]FIG. 8 is a flow diagram of an alternative message sequence forreducing the total power amount of forward traffic channel transmittedfrom base stations in an active set when excess power is beingtransmitted;

[0037]FIG. 9 is a diagram of a multi-carrier forward link;

[0038]FIG. 10 is a block diagram of a multi-carrier forward linktransmitter; and

[0039]FIG. 11 is a block diagram of a multi-carrier forward linkreceiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Referring now to the drawings, wherein like reference numeralsdesignate identical or corresponding parts throughout the several views,and more particularly to FIG. 1 thereof, there is illustrated acommunication system 2 which is preferably a cellular telephone system,although equally applicable in a public branch exchange (PBX), personalcommunication services (PCS) system, satellite based communicationssystem, indoor wireless network or outdoor wireless network. The system2 uses code division multiple access (CDMA) modulation and demodulationtechniques in communications between system resources. A systemcontroller (selector) 10, commonly referred to as mobile telephoneswitching office (MTSO), includes interface and processing circuitry forproviding system control to a set of base stations 12, 14, 16, 17 and19. The system controller 10 also controls routing of telephone callsfrom a public switched telephone network (PSTN) to the appropriate basestations 12, 14, 16, 17 and 19 for transmission to the appropriatedestination. A connection to or from the PSTN may be wireless, opticalfiber, or “wired” communication (e.g., twisted pair or coaxial cable).The system controller 10 communicates with private and public networkswhich include data networks, multimedia networks, and other private andpublic communication entities. Furthermore, system controller 10communicates to and from other base stations which are not shown in FIG.1.

[0041] The system controller 10 communicates with the base stations 12,14, 16, 17 and 19 by various means such as dedicated telephone lines,optical fiber links, coaxial links, or by radio frequency (RF)communication links. The base stations 12, 14 and 16 communicate withother systems such as mobile station (“mobile”) 18 via single carrierwireless CDMA communications. Base stations 17 and 19 communicate withother systems such as mobile 21 via a multi-carrier link comprised ofthree CDMA signals illustrated by arrows 26 a-c. Mobile 21 communicateswith base stations 17 and 19 via single carrier reverse link 28. Itshould be noted that a multicarrier forward link may consist of morethan three carriers or it may consist of less than three carriers. FIG.1 also illustrates a multicarrier and a more conventional single carrierdirect spread system coexisting in the same system. It should be notedthat while this is possible, it is preferable that a system use only asingle type for forward link.

[0042] Arrows 20 a and 20 b illustrate the respective reverse andforward links between the base station 12 and the mobile station 18.Arrows 22 a and 22 b illustrate the reverse and forward links betweenthe base station 14 and the mobile station 18. Similarly, arrows 24 aand 24 b illustrate the possible reverse and forward links between thebase station 16 and the mobile station 18. While crosslinks between therespective base stations 12, 14, 16 are not shown in FIG. 1, or a director radio frequency connection from controller 10 to the mobile 18, suchpossibilities are included within the inventive aspects of the presentinvention.

[0043] Base stations 12, 14, and 16 each transmit traffic data via aWalsh code channel to the mobile 18 on the communication forward links20 b, 22 b, and 24 b, when the system controller 10 assigns basestations 12, 14, and 16 to the mobile's active set and instructs therespective base stations to establish an interface with that mobile 18.The code channel allocated for communication with a mobile 18 is alsoreferred to as a traffic channel. Each of the code channels transmittedfrom different base stations to the mobile contains redundantinformation and is available to the mobile 18 to combine the respectivecode channels using a diversity combining mechanism (explained in moredetail herein). To increase the forward link rate to a mobile, multiplecode channels may be used from the same base station. In this case, theaggregation of code channels is called the traffic channel. The forwardlink signal includes the aggregation of the code channels including theset of traffic channels and the additional control channels such as thepilot, synchronization and paging channels. The present inventionreduces the transmit power of the forward link signal by reducing thetime the traffic channels are active during a soft handoff.

[0044] Base stations 12, 14 and 16 also respectively transmit pilotchannel to the mobile 18 along forward communication links 20 b, 22 band 24 b. The pilot channels are distinguished from the traffic channelstransmitted from the same base station by different Walsh codes. Therespective pilot channels from different base stations are distinguishedfrom one another by pilot PN code shifts. In the absence of blockage orfading, the pilot channel received at the mobile 18 from the basestation 16 would expect to be larger in received signal power than thatof base stations 12 or 14 because the mobile 18 is closest to the basestation 16.

[0045] Alternatively, in lieu of a separate code channel (Walsh code)for the pilot, the pilot can be embedded or multiplexed into the trafficchannel streams which are sent to individual mobile stations. Theembedding can be done by using special pilot symbols, or an auxiliarysignal. When the embedded pilot is used, there is typically a commonpilot which is used for initial acquisition of the system and fordetecting when to handoff. Alternatively, separate pilots can betransmitted on a per traffic channel basis or per group of trafficchannels.

[0046] When the mobile 18 is in a soft handoff region (e.g., when movingfrom a coverage region of at least one base station to at least oneother base station) the system controller 10 dispatches a handoffdirection message that includes a list of base stations assigned to themobile's active set. The handoff direction message may also includeauxiliary information, such as handoff thresholds (e.g., add thresholdand drop threshold) which is useful to the mobile station afterperforming the handoff. As described in the above referencedapplications and in the IS-95 standard, the active set contains pilotsfrom base stations with which an interface to the mobile has beenestablished. The candidate set contains pilot channels that haverecently been detected with a sufficient strength by the mobile, and thecandidate set contains pilot channels from base stations known to be inthe same geographic region.

[0047] Knowing which pilot channels will likely have a reasonablestrength (i.e., by knowing which base stations are assigned to themobile's neighbor and candidate set), the processing required at themobile is reduced in that the mobile may search more frequently for thepilot channels corresponding to the base stations in the mobile'sneighbor and candidate sets, as well as in the active set.

[0048]FIG. 2 is a graph showing relative pilot channel quality that maybe observed by the mobile 18 from cells 12, 14 and 16, as shown inFIG. 1. The graph in FIG. 2 plots energy per PN chip (Ec) per totalreceived power (Io) at the mobile 18 vs. time for three exemplary pilotchannels from base stations 12, 14, and 16. As shown in FIG. 2, thepilot from base station 16 degrades in signal quality with increasingtime, indicating the mobile 18 is moving away from base station 16.Conversely, the pilot from base station 12 improves in signal qualitywith time, implying that the mobile 18 is moving toward base station 12.The pilot from base station 14 remains relatively constant in signalquality, indicating the mobile 18 is moving along a coverage perimeterof base station 14.

[0049] The area of interest in FIG. 2 is the soft handoff region. In thesoft handoff region, the mobile 18 and the system controller 10communicate with one another to determine which base stations should bewithin the mobile's active set based on the relative pilot channelqualities of the cells 12, 14 and 16. In the illustrative example, thepilot channel from base station 16 is originally in the mobile's activeset because base station 16's pilot channel level is above the addthreshold level. However, at the end of the soft handoff region, thepilot from base station 16 drops below the drop threshold level for someperiod of time.

[0050] In response, base station 16 is caused by the system controller10 to be dropped from the active set by the mobile communicating to thesystem controller 10, via a pilot strength measurement message. Becausethe pilot from base station 14 never surpasses the add threshold level,base station 14 is not added to the active set. In contrast, basestation 12 surpasses the add threshold level for the necessary period oftime, and thus is added to the active set as determined by the systemcontroller 10 in response to a pilot strength measurement messagegenerated by the mobile 18. Toward the end of the soft handoff region,only base station 12's signal remains within the mobile 18's active set.

[0051] Often, the poorly received pilot channel is detected above thedrop threshold with sufficient frequency to keep the corresponding basestation in the active set, even though the corresponding traffic channelcontributes little to the reception quality at the mobile. This isparticularly true in a slow fading environment. In the case of a slowfading environment, the received signal levels from the base stationslowly change relative to each other. Typically one base station isstronger than another for a while and vice-versa. The fading rate is notsufficiently fast enough to obtain the short-term benefit of diversity.Thus, it would be preferable to transmit from the stronger base stationand not from the weaker base station.

[0052] The present invention seeks to reduce the transmission time ofthe code channels from some base stations in a fading environment todecrease the total transmit energy generated for the associatedcommunication. Reducing the total transmit energy of a particularcommunication improves overall system capacity. It should be noted thatone could use handoff procedures which would remove the base stationsfrom the active set, thus reducing the transmit power. However, thisapproach requires considerable signaling in the infrastructure and isthus relatively slow. This makes it difficult to quickly switch totransmitting from another base station when its signal becomes thestronger signal.

[0053] Another case in which this invention provides benefit is when onebase station is received at the mobile station at a lower signal levelthan the other base station, but is still above the drop threshold. Inan environment with little fading, it is preferable to transmit onlyfrom the base station whose signal is being received more strongly atthe mobile station. However, removing the base station from the activeset and then using the handoff procedures to restore it to the activeset adds considerable delay in the case that this pilot becomesstronger. This delay reduces the quality of the link and can result indropped calls.

[0054]FIG. 3 is a block diagram of the mobile 18. An antenna 30 iscoupled through a diplexer 32 to an analog receiver 34 and transmitpower amplifier 36. The diplexer 32 cooperates with the antenna 30 suchthat simultaneous transmission and reception is achieved through theantenna 30. While receiving RF energy from the respective base stations12, 14, and 16 (FIG. 1), antenna 30 receives transmitted pilot and codechannel signals routed through diplexer 32 to the analog receiver 34.The analog receiver 34 receives the RF energy from diplexer 32 andimplements an open loop power control function for adjusting thetransmit power of the mobile station for transmissions on a reverse link(i.e., mobile to base station). More particularly, the receiver 34generates an analog power control signal that is provided to a transmitpower control circuit 38, as is discussed in U.S. Pat. No. 5,056,109,entitled “Method and Apparatus for Controlling Transmission Power in aCDMA Cellular Mobile Telephone System”, assigned to the assignee of thepresent invention, and incorporated herein by reference. A closed looppower control adjustment is developed by control processor 46 using areverse link power control bit stream which was transmitted on theforward link and demodulated by digital data receivers 40, 42, and 45.The analog receiver 34 converts the received RF energy into a basebandsignal and digitizes the baseband signal.

[0055] The digitized output from the analog receiver 34 is provided to asearch receiver 44 and digital data receivers 40, 42, and 45, whichoperate under the control of control processor 46, receive code channelsfrom respective base stations, and provide respective outputs to adiversity combiner/decoder 48. The diversity combiner/decoder 48combines the respective output signals from the receivers 40, 42 and 45based on a selected combining scheme, discussed later in more in moredetail.

[0056] While three digital data receivers 40, 42 and 45 are shown inFIG. 3, the diversity combiner/decoder 48 is typically equipped tointerface with a number of additional digital data receivers.Preferably, the number of digital data receivers included in mobile 18is equivalent to the maximum number of code channels (accounting forseparate direct and multipath signals produced from each code channel)that the mobile will employ in its combining scheme. As will bediscussed, additional diversity gain is possible with the inclusion ofadditional data receivers, and the present invention is applicable toany number of digital data receivers (or signal multichannel digitaldata receiver).

[0057] The digital data receivers 40, 42 and 45 cooperate with thediversity combiner/decoder 48 to form a “rake” receiver structure. Thediversity combiner/decoder 48 cooperates with each of the respectivereceivers 40, 42 and 45 serve as three “fingers” in a rake. Moreparticularly, the receivers 40, 42 and 45 may be set by the controlprocessor 46 to receive code channels from different base stations, oran multipath signal from a common base station. Thus, all threereceivers 40, 42, and 45 may be used to receive code channels from threedifferent base stations, or a signal code channel from a base stationwhich arrives via three different signal paths (i.e., three multi-pathsignals). It should be clear that the receivers 40, 42, and 45 may beused to receive any combination of multipaths and code channels fromdifferent base stations. The rake receiver structure may also beimplemented in numerous other configurations based on, for example,several single channel receivers, multichannel receivers (i.e., havingat least one channel) and diversity combiner combinations. Furthermore,the diversity combiner function could be incorporated into controlprocessor 46, or one of the receivers 40, 42, 44, and 45.

[0058] In the preferred embodiment, the output of the diversitycombiner/decoder circuit 48 is passed to a deinterleaver and a decoder.The output of the decoder is typically passed through a control unitwhich splits apart the received data stream into end user data andcontrol data. The end user data is supplied to a data device, such as aspeech coder.

[0059] The data output of a data device, such as a speech codec is to betransmitted on the reverse link to the base stations in the mobilestation's active set. The output of the user digital baseband circuit 50is a baseband signal which is formatted, encoded, interleaved, and ispassed to a transmit modulator 52 where it is modulated. An output ofthe transmit modulator 52 is passed through a transmit power controldevice 38 under control of the control processor 46. The transmit powercontrol circuit 38 adjusts the output power of the mobile 18 (FIG. 1),based on the power level signal provided by analog receiver 34 andclosed loop power control bits, and an output RF signal is passed to atransmit power amplifier 36 which amplifies the output signal and passesthe amplified output signal through a diplexer 32 and transmittedthrough antenna 30.

[0060] The digitized IF signal from the analog receiver 34 contains thecode channel signals and pilots transmitted by the base stations in thepilot's active set along with other CDMA signals which act asinterference to the mobile 18. The function of the receivers 40, 42 and45 is to correlate IF samples with the proper PN sequence. Thiscorrelation process provides the “processing gain” which enhances thesignal-to-interference ratio of the signal intended for the mobile bymatching the PN sequence used in the respective code channels to encodethe message being sent to the mobile. Unintended signals that have notbeen encoded with the matching PN sequence are “spread” by thecorrelation process, thereby decreasing the signal-to-interference ratiofor the unintended signals. The correlation output is coherentlydetected using the pilot carrier as a carrier phase reference. Theresult of this detection process is a sequence of encoded data symbols.

[0061] Search receiver 44, under control of the control processor 46,scans for received pilot channels and multi-path pilot channels from thebase station via direct paths and reflected paths (e.g., multipaths).The search receiver 44 uses a ratio of the received pilot energy perchip (Ec) to total received spectral density, noise and signals, denotedas E_(c)/I_(o), as a measure of the quality of the received pilot.Receiver 44 provides a signal strength measurement signal to the controlprocessor 46 indicative of the respective pilot channels and theirstrengths.

[0062] The diversity combiner/decoder circuit 48 adjusts the timing ofthe inputted received signals into alignment and adds them together.This addition process may be preceded by multiplying the respectiveinputted signals by a weighting factor corresponding to the relativesignal strengths of the pilot channels corresponding to the respectiveinputs. The weighting factor is based on the pilot strength because itis presumed the respective signal quality of each pilot corresponds withthe signal quality of the signals transmitted on the respective basestations' code channel. When using the weighting factor, the combinerimplements a maximal ratio diversity combining scheme. The resultingcombined signal stream is then decoded using a forward stream errordetection decoder that is also contained within diversitycombiner/decoder circuit 48. The pilot based weighting method works wellwhen the base stations in the active set transmit the code channelsignals to the mobile station in equal proportion to the pilot signal.That is, the ratio of code channel power to pilot power is the same inall members of the active set. If the ratio is not the same, then otherweighting methods may be preferable. For example, the base station maysend to the mobile station, in a signaling message or by some othermeans, the ratio of traffic channel to pilot channel power being used byall base stations in the active set. Then if the relative fraction forbase station j is α_(j), mobile station can combine code channels usingweights {square root}{square root over (α_(j)γ_(j))} where γ_(j) is therelative received power of the pilot for base station j at the mobilestation. Alternatively, the mobile station may estimate α_(j) orα_(j)γ_(j) from the received signal from base station j.

[0063] Baseband circuitry 50 includes voice coder (vocoder) datainterfaces and other baseband processing features. In addition, userdigital baseband circuit 50 interfaces to I/O circuits such as a handsetwhich inputs a voice signal to a digitizer and vocoder (voice coder)contained therein. The output of the user digital base band circuit 50is provided to a transmit modulator 52 which modulates an encoded signalon a PN carrier signal whose PN sequence corresponds to an assignedaddress function for the outgoing call. This PN sequence is determinedby the control processor 46 from call setup information that istransmitted by the base station (12, 14 or 16) and decoded by thereceivers (40, 42 or 45).

[0064] The output of the transmit modulator 52 is provided to thetransmit power control circuit 38 where the signal transmission power iscontrolled by the analog power control signal provided from the receiver34. Furthermore, control bits are transmitted by the base stations inthe form of power adjustment commands, to which the transmit powercontrol circuit 38 is responsive. Transmit power control circuit 38outputs the power control modulated signal to transmit power amplifiercircuit 36, which amplifies and converts the modulated signal to an RFfrequency. Transmit power amplifier 36 includes an amplifier whichamplifies the power of the modulated signal to a final output level. Theamplified output signal is then passed to the diplexer 34 which couplesthe signal to the antenna 30 for transmission to the base stations 12,14 and 16. Signals intended for the system controller are received bythe base stations 12, 14, and 16 and respectively passed to the systemcontroller 10 where they are combined.

[0065]FIG. 4 is a graph of a diversity receiver performance, measured inprobability of frame error rate, versus Eb/No where the diversityreceiver implements maximal ratio combining. Four exemplary curvesrepresentative of probability of frame error rate are shown respectivelyrepresenting a mobile receiver having one finger (M=1), two fingers(M=2), three fingers (M=3), or four fingers (M=4) configured to receivesignals from a corresponding number of a base stations. Comparing thecurves for M=1 and M=2, the performance of a receiver having two fingersand processing two paths is better than that for receiver processing onepath. This comparison is made by observing, for a given frame error rate(i.e., the dashed line), a distance between the respective probabilityof frame error curves. In the exemplary graph, a performance improvementis shown by the distance M₁₋₂. Similarly, if a diversity receiver havingthree fingers is used by the mobile, a performance improvement of M₂₋₃is achieved where, generally, M₂₋₃ is less than the performanceimprovement of M₁₋₂. Similarly, adding a fourth finger to a diversityreceiver, provides a performance improvement as shown by M₃₋₄. It shouldbe noted that M₃₋₄ is less than M₂₋₃ and M₁₋₂. Thus, if the mobile werethe only mobile in the CDMA system, diversity receivers having anincreasingly large number of fingers receiving a corresponding number oftransmissions from base stations, would provide continuing improvedperformance, albeit, the improvement reaching de minimis returns for Mbeing a large number. Furthermore, the aforementioned performancerelationship presumes none of the fingers contribute only noise (orpractically only noise) to the combining process. Absolute amounts ofimprovement depend on the communications conditions (e.g., amount offading, type of fading, impulsivity of noise, proximity to base station,etc.).

[0066] During soft handoff, system capacity is affected differently byexploiting diversity combining processes on the forward link and on thereverse link. For example, on the reverse link, the mobile transmits tothe base stations 12, 14, and 16 through paths 20 a, 22 a, and 24 a(FIG. 1) respectively. Each of the base stations receive thetransmission from the mobile 18, and forward the same to the systemcontroller (selector) 10, which combines the respective signals providedby the base stations 12, 14, and 16 using a diversity combining process.Because only one mobile 18 is transmitting, the system capacity is notadversely affected by the use of diversity combining.

[0067] On the forward link, however, the mobile 18 combines differentsignals (all having the same encoded information) transmitted from basestations 12, 14, and 16. Various methods for combining are known in theart including maximal ratio combining, equal gain combining, and simpleselection whereby one signal is selected for processing and the othersignals are discarded. Providing an additional, and perhaps excessive,number of base stations to the mobile's active set will certainlyimprove the performance observed at that mobile, but may actuallydegrade an overall system capacity of the CDMA system, since additionaltransmissions from the base stations communicating to the first mobilewill appear as background interference to a second mobile. Theusefulness of a particular code channel depends on a variety of factors,including its strength relative to the code channels from other basestations.

[0068] The total power radiated in the CDMA communications system istypically smaller if there is a sufficient gain in diversity. However,as recognized according to the present invention, the total power whichis radiated is typically larger than what is required for adequateperformance even if the additional diversity is not needed. Whether anincrease or decrease in an amount of radiated power from each of thebase stations is affected, depends on characteristics of thetransmission paths between the base stations and the mobile station. Inaccordance with one embodiment of the invention, the total transmitpower from the CDMA system is set to a more optimum operating point byincreasing the coordination between the mobile 18 and system controller(selector) 10. A description follows on how to collect at the mobile theinformation needed so the system can operate at a higher capacity.

[0069]FIG. 5A is a graph of Ec/Io vs. time for a soft handoff region inwhich three pilots A, B, and C from respective base stations areincluded in the mobile's active set. During the soft handoff region, asseen from FIG. 5A, changes in the respective communication channels forpilots A (shown by a dotted line), B (shown by a dashed line), and C(shown by a solid line) cause variations in signal strength and thussignal-to-noise ratios which cause respective pilots A, B, and C tofluctuate. It is these fluctuations that offer significant potential forimproving diversity gain, and the present invention teaches how toexploit the diversity gain so to maximize system capacity by changingforward traffic channel power allocation in rapid fashion.

[0070] The relative pilot quality strengths (pilot quality) of thepilots A, B, and C fluctuate from frame to frame, and as seen from FIG.5A, any one of the signals A, B, and C varies in SNR relative to theother signals. For example, in the first frame, pilot A provides thegreatest SNR while pilot B provides the least SNR. However, in frame 2,the relative signal-to-noise ratios of pilots B and C cross (as shown inFIG. 5A) and at the end of frame 2 the SNR of pilot B is greater thanthat of pilot C.

[0071]FIG. 5B is identical to FIG. 5A, but includes a level Δ_(r) (shownas a crossed “x” line), calculated by control processor 46 (FIG. 3) ofmobile 18, where Δ_(r) is representative of a fixed level Δ beneath thestrongest signal-to-noise ratio of the pilots A, B, and C in themobile's active set. Preferably, Δ_(r) is a single number generated bythe control processor 46, although variations of Δ_(r) (i.e., aplurality of Δ's) may alternatively be employed, such that gradations ofΔ are used to resolve more finely the relative signal qualities of thepilots. Control processor 46 calculates a threshold signal Δ_(r)preferably continually, although an alternative piecewise or discreteimplementation Δ_(r) may be produced.

[0072] As shown in FIG. 5B during the first frame, only the pilot Δ isat or above the threshold signal

_(r), which, in this example, is set by the pilot A itself (i.e., pilotA has the strongest SNR and thus Δ_(r) is based on a level ΔdB beneaththe SNR set by pilot A). It is also noted that signals B and C are notat or above signal level Δ_(r). Accordingly, FIG. 5B indicates in frame1 that pilot A (as denoted by the character “A” written over top of the“TIME” axis in the first frame) is at or above the signal Δ_(r) and hasthe greatest average SNR over that past frame interval. In frame 2, thestrongest SNR is that of signal A, followed by pilot B, and the lowestpilot is C, all of which are above Δ_(r) at the end of the frame. Inframes 3 and 4, only pilots A and B are above Δ_(r). In frame 5, pilot Chas the strongest SNR (and thus Δ_(r) is calculated based on pilot C).Pilot A is then the next strongest signal and is greater than the SNR ofpilot B, all of which are above Δ_(r).

[0073] By calculating Δ_(r) and comparing Δ_(r) to each of therespective signals from the base stations in the active set, the mobilehas effectively gathered a significant amount of information regardingparticular communications channels within a given frame. Thischaracterization of the communications channels may be exploited by themobile by configuring the mobile's diversity receiver and combiner inorder to detect optimally the signals transmitted from the respectivebase stations. Additionally, in accordance with one embodiment of theinvention, the CDMA communication system performance is also optimizedby communicating the best signal qualities of the pilots within theactive set to the system controller on a frequent basis so the systemcontroller can make commensurate adjustments on the forward trafficchannel power allocation between the base stations in the active set.The information is rapidly communicated to the system controller 10(FIG. 1) because the optimum number and selection of transmitting basestations does not remain constant as the relative SNR's of the signalsfrom each base station change rapidly from frame to frame as illustratedin FIG. 5.

[0074] It should also be noted that the a value which is used to computeΔ_(r) could be pre-stored in the mobile station or it could be sent tothe mobile station via a signaling message or some other control method.It should also be noted that FIGS. 5A and 5B are described in thecontext of frames which may correspond to the frames used for dataframing, interleaving, and encoding on the traffic channel as describedin the IS-95 standard. However, this is not necessary in this inventionand the frames shown in FIGS. 5A and 5B may not correspond to anyparticular processing interval, and may be either shorter or longer thanthe exemplary value of 20 ms. Additionally, the various transmissionsdescribed above are generated by different base stations. However, theinvention is also applicable to any element radiating a forward linksignal. In particular, the invention applies to different antennas atthe same base station radiating the same signal. For example, thesignals A, B, and C in FIGS. 5A and 5B can be from different antennas ofthe same base station, as would be the case where there are threeantennas at one base station.

[0075] It should also be understood that the set of signals A, B and Cshown is FIGS. 5A and 5B can be from any combination of base stations orantennas at a base station. For example, signals A and B could be fromtwo different transmitting antennas at base station 17 and signal Ccould be transmitted from base station 19. Signals A, B, and C could bemulti-carrier forward links all transmitted from the same base station,or could be the signals from different antennas radiating themulti-carrier forward link. For example, if base station 17 transmittedthree carriers from two antennas, then signal A could consist of twocarriers and signal B of one carrier. Signal A would be comprised of twodifferent separate carrier signals, however, in this example, both thesecarriers are radiated from the same antenna and will be received by themobile station ]essentially at the same level, provided that they aretransmitted at the same level. It should also be clear that in a realsystem there may be many more than three signals (which are show inFIGS. 5A and 5B) which the mobile station is tracking.

[0076] To provide the system controller 10 (FIG. 1) with thisinformation on a rapid basis, the present invention provides a novelcommunications protocol between a mobile and system controller 10discussed herein in reference to FIGS. 6A-6C. FIGS. 6A-6C show alternateforms of a signaling or control messaging in the form of a bit vectormessage reported to the system controller (selector) 10 through thereverse link signal transmitted from the mobile 18 to the selector 10 byway of one or more base stations (12 and 14). The bit vector message ispreferably transmitted on a frame-by-frame basis, although more frequentreporting, as well as less frequent reporting, are alternatives.

[0077] In one embodiment of the invention, a multichannel reverse linksignal is employed, where the reverse link signal is comprised of a setof orthogonal code channels defined by a set of Walsh codes in similarfashion to the forward link. In this multichannel reverse linkimplementation, the bit vector message is preferably communicatedthrough one of the orthogonal code channels in the reverse link, so tominimize the delay time before the system controller can act on theinformation contained in the bit-vector message. A system and method fortransmitting data using such a reverse link signal is described inissued U.S. Pat. No. 5,930,230 entitled “HIGH DATA RATE CDMA WIRELESSCOMMUNICATIONS SYSTEM” issued Jul. 27, 1999, assigned to the assignee ofthe present invention and incorporated herein by reference.

[0078] In an alternative embodiment of the invention, a single codechannel reverse link signal is employed, as is used in an IS-95compliant system. The bit-vector message is preferable transmitted alongwith the other user data within the single code channel via timemultiplexing or bit puncturing the data vector into the reverse link PNcode.

[0079]FIG. 6A shows a data structure for pilot quality bit-vectormessage generated by the mobile and transmitted to the system controller10 via the base stations. In particular, FIG. 6A shows a 10-bit vectormessage which is short in length, yet capable of reporting to the systemcontroller 10 which of the pilots in the mobile's active set have signalqualities at or above a given standard (e.g., the Δ_(r) threshold signalin FIG. 5B). The bit-vector message need not be limited to 10-bits, andcan be in other formats other than a bit-vector although it is desirableto have a short message. In order to reduce the number of transmittedbits, the bit-vector message presumes an arrangement of the respectivepilot channels based on an initial ordering of the pilots identified tothe mobile from the system controller in a handoff direction message.

[0080] The CDMA IS-95 standard permits up to six members (pilots) in theactive set, all of which can be accommodated in the pilot qualitybit-vector message. In FIG. 6A, the pilot having the best (i.e., highestsignal to interference ratio) as judged by the process described inreference to FIG. 5B, is identified by a three-bit data field indexwhich uniquely identifies its position as originally reported to themobile in the handoff direction message. The index is denoted in FIG. 6Aby the three-bit data field I₁, I₂, and I₃. Thus, if the pilot channelfrom the second base station reported to the mobile in the last handoffdirection message is received with the greatest SNR, the three bit indexis set to two (binary 010), or alternatively 1 if the index runs from 0to 8.

[0081] The bit fields U¹, U², U³, U⁴, U⁵, and U⁶ each refers torespective pilots as originally listed in the handoff direction message,and indicates whether the corresponding pilot channel was received abovethe Δ_(r) threshold signal. For example, the bit in the data fields U¹⁻⁶is set to 1 (or alternatively 0) indicating to the system controller 10that the pilot channel corresponding to that bit position is beingreceived equal to or above the Δ_(r) threshold signal. In particular, ifU¹ is set to a 1, the system controller 10 would recognize that thefirst pilot identified in the last handoff direction message has asignal-to-noise ratio at the mobile as being equal to or above Δ_(r), ascalculated by control processor 46. U²⁻⁶ are also set by the processor46 preferably on a frame-by-frame basis and transmitted to the systemcontroller 10 via the base stations in bit-vector messages.

[0082] The last element of the data field, H^(m), is the sequence numberof the handoff direction message. The data field H^(m) is used toprovide the system controller 10 with an identification of the activeset to which the mobile is referring. H^(m) could be several bits inlength; alternatively it could be a single bit. For the single bit case,H^(m) could be the last bit of the sequence number. Thus, if the basestation sent handoff direction messages with sequence numbers equal to‘100’ followed by ‘101’ binary, then the mobile station would return ‘1’in H^(m) if it were referring to the handoff direction message withsequence number ‘101’ and would return ‘0’ in H^(m) if it were referringto the handoff direction message with sequence number ‘100’. Byincluding the sequence number, the base station can positively determinewhich pilot the mobile station is referring in the three-bit data fieldI₁, I₂, and I₃ and in the set U¹, U², U³, U⁴, U⁵, and U⁶.

[0083] In an embodiment of this invention which includes a multi-carrierforward link, the bit vector U¹, U², U³, U⁴, U⁵, and U⁶ can be expandedto N×M bits, where there are N possible base stations in the active setand there are M possible antennas at a base station. Alternatively, Mcan correspond to the number of possible multi-carrier forward links ata base station. In this embodiment, the mobile station reports thestrongest of the N×M multi-carrier forward links with the vector I₁, I₂,and I₃ (which may need to be longer to take into account the need toidentify the largest of N×M items) and reports which other multi-carrierchannels are above Δ_(r) using the vector U^(i). In an alternativeembodiment, the mobile station reports the strongest base station,rather than the strongest carrier, using vector I_(i) and then reportswhich other multi-carrier channels are above Δ_(r) using the vectorU^(i).

[0084] It should be noted that Δ_(r) can be either with respect to thestrongest base station or the strongest carrier over all base stationsin the mobile station's active set. It should further be noted that thestrongest base station can be determined by summing the pilot Ec/I_(O)'sfrom all the forward link carriers of a multi-carrier base station ashas been done with multipath components from the same carrier as iscommonly used in IS-95. Thus, the total strength of a base station isgiven by summing Ec/I_(O)'s from all the forward link carriers and allmultipath components on a particular carrier.

[0085] In response to the bit field message, the system controller 10receives the measured power message and, as will be discussed herein,determines which of the signals in the active set to remove from theforward traffic channels, and which of the base stations to keeptransmitting. That is, system controller 10 identifies which basestations are transmitting signals that are being received below theΔ_(r) threshold signal using the bit field message. System controller 10then instructs the identified base stations to stop transmitting thetraffic channel directed to the corresponding mobile, which in turnreduces the transmit power of the forward link signal generated by thesebase stations. In an alternative embodiment, the base station, insteadof the system controller, can receive the message and determine whetherit is to transmit the forward link. This method lowers delay though itmay be less reliable when the mobile station is in soft handoff as allbase stations (or the base stations which should be transmitting theforward link) may not receive the reverse link transmission.

[0086] The base stations respond by not transmitting the traffic channelduring next frame of data directed to the corresponding mobile. Becausethe signals from the identified base stations are being received by themobile 18 with a significantly lower SNR than at least one other forwardlink signal, the increase in the error rate of the mobile will be smallrelative to the reduction in transmit power for the entire system. Whilethe identified base stations discontinue transmitting the trafficchannel, the signal processing resources within those base stations willremain allocated and ready to begin transmitting the traffic channelupon request by system controller 10. Also, these base stationspreferably continue to process the reverse link signal transmitted frommobile 18.

[0087] As the communication continues, mobile 18 continues to monitorthe relative strength of the pilots received from the base stations inthe active set. When the status of a pilot changes, for example when apilot is received above the Δ_(r) threshold, mobile 18 generates anotherbit field message indicating this change in status. Mobile 18 alsogenerates a bit field message when the pilot channel with the best SNRchanges. System controller 10 receives the bit field message andinstructs any base station in the active set for which the status haschanged to either begin transmitting the traffic channel for thatmobile, or to discontinue transmission of the traffic channel, as thecase may be. Each base station responds by transmitting the next dataframe via the traffic channel if the instruction was to begintransmission, or by not transmitting the next data frame if theinstruction was to discontinue transmission of the traffic channel.

[0088] In alternative embodiments of the invention, mobile 18 generatesbit field messages periodically, for example once each frame. By keepingresources allocated within each base station for transmitting thetraffic channel, the traffic channel can quickly be activated anddeactivated in response to rapidly changing conditions.

[0089] In still another embodiment of the invention, the systemcontroller 10 includes a gain adjust field in each data frame sent to abase station. The gain adjust field indicates the transmit power gain atwhich the frame is to be transmitted from the base station. When systemcontroller 10 receives a vector indicating that the pilot channel from aparticular base station is received less than the Δ_(r) threshold belowthe strongest pilot channel, the gain adjust in the next frame directedto that subscriber is reduced. Subsequent frames can be reduced furtheras more vectors indicate the pilot channel from that base stationremains the Δ_(r) threshold below the strongest pilot.

[0090] Control system 10 may also perform a more advanced analysis ofthe bit vectors received to better determine the stability of theenvironment in which the mobile is operating. In particular, controlsystem 10 may monitor the rate at which a particular pilot channelchanges from being above and below the Δ_(r) threshold. If the rate ofchange exceeds a predetermined threshold, control system 10 willdetermine that the mobile is in a fading or otherwise unstableenvironment, and therefore that the signal from each base station in thesoft handoff should be transmitted continuously. When such adetermination is made, control system 10 instructs all the active setbase stations to continue transmitting the forward link traffic channel,even when some pilot channels are detected the Δ_(r) threshold below thebest received pilot channel.

[0091]FIG. 6B shows an alternative data structure for a pilot qualitybit vector message transmitted from the mobile to the system controller10 via the base station. This alternative embodiment is similar instructure to the data structure defined in FIG. 6A although onlyincluding five bits for identifying the six members of the active set.Only five bits are used because the identity of the sixth (i.e., thebase station providing the strongest signal-to-noise ratio) isidentified by the first three bits of the pilot quality bit vectormessage (i.e., I₁₋₃). By uniquely identifying the strongest signal inthe first three bits of the pilot quality bit vector message, each ofthe other members of the active set is sequentially identified by thesubsequent bits in the pilot quality bit vector message, with animplicit understanding that there is no bit identifying the position ofthe strongest base station.

[0092]FIG. 6C shows a further alternative pilot quality bit vectormessage format where the first three bits I₁₋₃ are used to uniquelyidentify the strongest pilots of the base stations in the active set,the next three bits, J₁₋₃, identifying the second strongest, and thethird set of three bits, K₁₋₃, identifying the third strongest pilot ofthe members of the active set. Thus, each of the three strongest pilotsof the members in the active set are uniquely identified. An extensionof this embodiment would be to add an additional three bits for each ofthe fourth, or fifth, or sixth strongest pilot from the members of theactive set thus uniquely identifying them. A further embodiment would beto add an additional bit to the message to indicate the relativestrength of the pilots in finer quantization levels, rather than merelyabove and below the threshold Δ_(r). A still further embodiment would beto include all the Ec/Io value for each pilot. Thus, for a system withsix possible pilots in the active set, the Ec/Io would be included foreach possible pilot in the active set. It should also be clear thatsending the Ec/Io of the largest pilot in the active set and thenrelative Ec/Io values relative to the largest pilot is another possibleembodiment. While each of the embodiments in FIG. 6A through FIG. 6Cdefine alternative ways to report the relative measured powerspreferably on a frame-to-frame basis, combinations of the alternativemethods are possible as well. For example, the first six bits of themeasured power message may be used to uniquely identify the first twostrongest pilots of the member base stations, while the next three bitsare used to identify the relative positions of the next strongest threepilots (i.e., for a set of five members).

[0093] A further alternative approach would be to have only a singlebase station transmit to the mobile station. In this case, only thethree bit vector message (i.e., I₁₋₃) needs to be sent from the mobilestation to the base station. An alternative arrangement is to have themulti-carrier base station transmit through only one antenna at a time.In this case, a single bit is needed to specify which antenna can beused. Clearly, this can be used in combination with the methodsdescribed above.

[0094] When communicating over known fast or slow fading channels, analternative embodiment for determining the Δ_(r) threshold is employedto more effectively overcome the effects of fading. In contrast to thepreferred embodiment where Δ_(r) is based on the pilot having thelargest average SNR over the frame, in the present embodiment theminimum value of the maximum pilot over the frame is used to determineΔ_(r). Thus, if at least the strongest pilots are subject to fading,setting the threshold Δ_(r) at the minimum of the strongest pilot overthe frame will permit more pilots to be above the Δ_(r) threshold.Accordingly, greater amounts of diversity gain can be achieved bycombining signals from more base stations thus adding more independentor at least semi-independent paths. More particularly, in a fast fadingenvironment, the above described use of the minimum value for thestrongest pilot over the frame is expected to work suitably well for afast fading scenario where fade durations are expected to be relativelysmall with respect to a frame's length.

[0095] However, for slow fading channels, the performance of the rakereceiver and the mobile is not as great as in the case of the fastfading environment, primarily because an interleaver used in the receiveprocess does not provide as much benefit as it ordinarily would when thefades have a duration which is less than the length of the interleaveduration. However, in slow fades where the duration of the fade isgreater than the interleaver span, a greater Eb/No is required in orderto provide acceptable communication quality at the mobile. Furthermore,the duration of one frame for performing an averaging on respectivepilot strengths is insufficiently short to determine whether or notrespective communication channels are subject to slow fading.

[0096] Accordingly, in this alternative embodiment each of therespective base stations implements a filter, which integrates andnormalizes each of the U_(k) bits (FIGS. 6A and 6B) in the bit vectormessage. If individual ones of the U_(k) bits toggle, i.e., changesstates at least once, then this toggling indicates the channel betweenthe respective base station and the mobile is subject to slow fading.Accordingly, system performance of the CDMA system will be improved ifthe base station subject to the slow fading continues to transmit on theforward traffic channel. This observed toggling may also be used as anindicator at the system controller to indicate whether the mobile shouldbe placed in a soft handoff region. For example, if the bit fieldrepresenting the pilot strength for a given base station is nearlyalways 0, or always 0, then the respective base station should indicatethat the pilot is in fact much weaker than the strongest pilot, and thebase station producing the weaker pilot should not be included in theactive set because it adds practically no beneficial value to theperformance of the mobile. It should also be clear that the mobilestation can effectively monitor the toggling operation and then transmitthe message to the base station only when it wants to change the basestations transmitting to the mobile station.

[0097] Another alternative allows the signaling and switching processesto take place more quickly. In this case, the mobile station signals abase station directly during fading when the signal from that basestation becomes either stronger or weaker than the signals from one ormore other base stations. The base station responds by not transmittingor not transmitting the next frame. In this case, the switching can bequite rapid because the base station can respond more quickly than thebase station controller, allowing a first frame to be sent from one basestation and the next successive frame to be sent from another basestation. This works at relatively medium fading rates. When thesignaling and switching is even faster, the switching can occur during aframe. In this case, the base station must receive the data to betransmitted during the frame. In one embodiment, the base stationsencode, interleave, and further process the data for transmission. Thedata output stream is enabled or disabled based upon the feedback fromthe mobile station.

[0098] As an alternative to the threshold method for determining whichpilots to identify in the pilot quality bit vector, a second “fingerassignment” method is herein described. In the mobile, the mobilestation makes estimates of the received pilot Ec/Io from every basestation in the active set. If the mobile does not have a finger of itsdiversity receiver assigned to the base station, the Ec/Io for thatpilot is set to 0. If the mobile station has a diversity receiver fingerallocated to a given base station, the mobile determines the averageEc/Io over the previous 20 milliseconds (preferably, although,alternatively, longer or shorter averaging times could be used) andreports that value. The 20 ms period corresponds to a CDMA frame length.The mobile station then identifies the largest pilot having the largestEc/Io value and assigned an index A^(m). For all other pilots in theactive set, the mobile station sets respective bit values in thebit-vector message to 1 if the Ec/Io value for that pilot is withinΔ_(r) of the Ec/Io value for the maximum pilot. If the receiver has onlyN fingers, where N is less than 6, then no more than N pilots arereported in the bit-vector message.

[0099] Because fingers may be assigned to both a direct signal path andan image path (i.e., a multipath image), the finger assignment methodprevents “too many” base stations from being reported as having signalsthat are usable by the mobile. For example, if a diversity receiver hasthree fingers, and only two base stations produce the three highestquality signals (i.e., the direct paths from each base station and animage signal), then there is no need for a third base station totransmit to the mobile, because the receiver does not have enoughfingers to receive it. On the other hand, if the pilot from the thirdbase station periodically surpasses one of the other three signals, themobile may nonetheless report all three stations as being above thedesired threshold, because there are a number of instances where thediversity receiver would combine the signal from the third base station.Thus, in one embodiment of the invention, the pilot SNR for a basestation is reported based on the finger with the highest SNR receivedfrom that base station.

[0100]FIG. 7 is a flowchart showing a preferred method for adjustingforward channel power allocations. The process begins in step S1 where amobile measures the pilot strengths (signal qualities) of all pilotswithin the mobile's active set. The process then proceeds to step S3where the mobile, based on the measured pilot strengths, measured instep S1, generates a threshold signal Δ_(r). The signal Δ_(r) isgenerated based on the pilot having the greatest SNR as measured in stepS1. The process then proceeds to step S5 where each of the respectivepilots, pilots, are compared with signal Δ_(r) to determine whether therespective pilot_(i) is greater than or equal to Δ_(r). This comparisonstep is performed, preferably, over the duration of a 20 millisecondframe period, and terminating at the end of a frame period, althoughother sampling intervals taken at other points within a frame or inmultiple frames is consistent with this embodiment. If the respectivepilot_(i) is greater than or equal to Δ_(r), a bit in the bit-vectormessage (see e.g., FIGS. 6a-6 c) indicating that the respectivepilot_(i) is greater than the threshold Δ_(r). If however in step S5 itis determined that pilot_(i) is not greater than or equal Δ_(r), a bitin the bit-vector message is set to indicate that the respectivepilot_(i) is less than or equal to Δ_(r) (preferably setting the bit to“0”).

[0101] After the pilot quality bit vector is formed in step S7 or instep S9, the process proceeds to step S11 where the mobile sends thebit-vector message to the base stations in the mobile's active set. Atthis time, the mobile sets a timing loop, which is used at the mobile asan indicator for the mobile to determine when the mobile should adjustits fingers based on the mobile's anticipation of the system controller10 adjusting the power in the forward traffic channel in response to themobile's earlier bit-vector message. By setting the timing loop (whichis easily accomplished by the mobile counting consecutive 20 ms frames),the mobile knows when the change in the forward traffic channeltransmissions will occur. After step S11, the process then proceeds tostep S13 where the base stations receive and relay the pilot quality bitvector to the system controller. After step S13, the process proceeds tostep S15 where a selector at the system controller processes thebit-vector message and produces a control message sent to each of therespective base stations in the mobile's active set controlling which ofthe base stations in the mobile's active set should transmit arespective code channel to the mobile. By controlling transmissions fromeach of the base stations in the mobile's active set, the total powerradiated from the base stations in the mobile's active set is reduced.

[0102] The process then proceeds to step S17 where after the timerreaches a time threshold, the mobile adjusts the fingers in itsdiversity receiver corresponding with the base stations identified asbeing equal to or greater than the signal Δ_(r) as determined in stepsS7 and S9. By adjusting the fingers, the mobile combines received energyonly from those base stations in the mobile's active set which in factare transmitting on their respective code channels. After step S17, theprocess repeats, where the mobile continues to monitor the respectivepilot strengths for each of the base stations in the mobile's activeset.

[0103] Because the mobile station generated the particular bit-vectormessage and the response of each base station to the bit-vector messageis based on a predetermined algorithm, the time at which each basestation changes the forward link allocation is known by the mobilestation. Thus, the mobile station can properly combine the signals fromonly those base stations which are transmitting at the time. This isadvantageous because combining the signals from base stations that arenot transmitting to the particular mobile station would causeunnecessary noise to be introduced into the receive processingnegatively impacting the result. This would result in a performanceloss, a higher required Eb/No and a loss in capacity. Similarly, if themobile station did not combine signals which were being transmitted tothe mobile station, and which were received with sufficient strength,there would be a loss in capacity.

[0104] In one embodiment of the invention, the mobile stationcompensates for transmission errors in receipt of the bit-vectorreceived by each base station by first attempting to demodulate thereceived forward frame assuming that the message was correctly receivedand processed by the base station. In most cases the mobile station willcorrectly demodulate the frame. However, if the frame is in error, thenthe mobile station can attempt to use the set of base stations that weretransmitting to the mobile station before it sent the most recentbit-vector message. Thus, if the base station did not receive the mostrecent bit-vector message, then the mobile station would attempt todemodulate the frame again using the set of base stations which waspreviously used. This requires that the mobile station maintain thereceived signal from the different set of base stations in a buffer.Then the mobile station would use the data in this buffer when there wasan error. This error correction processing is illustrated by optionalsteps S19 and S21 of FIG. 7, as indicated by the dashed line to stepS19.

[0105]FIG. 8 is a flowchart of an alternative method for changingforward traffic channel power allocation for the base stations in themobile's active set. The process begins in step S32 where the mobilemeasures the respective pilot strengths of each of the base stations inthe mobile's active set. Next, in step S34 the mobile generates thethreshold signal Δ_(r) based on the measured pilot strengths. Then instep S36, the mobile compares both the direct (direct_(i)) and multipathsignals, for each of the respective base stations and compares thedirect and/or multipath signals to determine whether either the director multipath signals are greater than or equal to Δ_(r). If a direct ormultipath image is greater than or equal to Δ_(r), the process proceedsto step S38 where the diversity receiver assigns a finger or fingers tothe direct and/or multipath signals that are greater than Δ_(r), asdetermined in step S36. Subsequently, the process then proceeds to stepS42. However, if in step S36 it is determined that neither the directnor multipath signals of a respective base station is greater than orequal to Δ_(r), the process proceeds to step S40 where none of thefingers of the rake receiver and combiner circuitry are assigned to thatparticular base station. The process then proceeds to step S42. Itshould be noted that the Δ_(r) in FIG. 8 is different from that in FIG.7. In FIG. 7, Δ_(r) was used to determine whether to report a pilot; inFIG. 8 it is used to determine whether to assign a finger of the rakedemodulator. As such, the Δ_(r) in FIG. 8 will typically be smaller thanthat of FIG. 7.

[0106] In step S42, the mobile sends a bit-vector message to the basestation and the active set, indicating the finger assignment made at themobile on the direct and the multipath signals. If either the direct orthe multipath signals are greater than Δ_(r), the mobile formats thebit-vector message indicating that at least the direct or the multipathimage is greater than or equal to Δ_(r). The process then proceeds tostep S44 where the base station relays the bit-vector message to theselector at the system controller such that the system controller isinformed of the finger assignment used at the mobile, and thus, canadjust the forward traffic channel power allocation of which basestations are transmitting to the mobile station for each of the basestations in the mobile's active set. The process then proceeds to stepS46 where the selector sends a control message to the base stations inthe mobile's active set indicating which of the base stations are totransmit on their respective code channels corresponding to the fingerassignment set by the mobile. The base stations relay the controlmessage to the mobile such that the mobile is notified that the basestations having been informed of the system controller's allocation ofthe forward traffic channel power. The process then proceeds to step S48where the mobile adjusts the fingers in the diversity receiver inresponse to the control message generated by the system controller.

[0107] It should be noted that either the control message sent from themobile station to the base station or from the base station to themobile station may be in error. A technique similar to that wasdescribed in conjunction with FIG. 7 can be used. In this case, if themobile station does not receive the control message from the basestation or if it receives a frame in error, it can demodulate previousset of base stations which were transmitting to the mobile station.

[0108] In an alternative method for changing forward traffic channelpower allocation, steps S1 through S15 are the same as that shown in thepreferred method of FIG. 7, although the base station also transmits tothe mobile an indication of which of the base stations are in facttransmitting on their respective forward traffic channels. Thus, in thisalternative embodiment, the system controller, not the mobile, controlswhich of the base stations transmits to the mobile.

[0109] This invention has been described in terms of setting a thresholdΔ_(r) relative to the strongest pilot as has been described in the textand FIGS. 5A and 5B. Many alternative metrics can be used. In particularone which sets the bit U^(i) to ‘1’ only when the pilot sufficientlyincreases the total Ec/I_(O) can also be used. This technique isdescribed in U.S. Pat. No. 6,151,502 entitled “Method And Apparatus ForPerforming Soft Hand-off In A Wireless Communication System” issued Nov.21, 2000, assigned to the assignee of the present invention andincorporated herein by reference.

[0110] The invention has been described in terms of transmitting theentire forward link from a set of base stations mobile station. A systemand method for conducting a high speed data link using a fundamental andsupplemental channel is described in co-pending issued U.S. Pat. No.5,987,326 “entitled Transmit Power Reduction For A High Speed CDMA LinkIn Soft Hand-off”, and in U.S. Pat. No. 6,173,007 entitled “High DataRate Supplemental Channel For CDMA Telecommunications System” bothassigned to the assignee of the present invention and incorporatedherein by reference. In this high speed data link system, the forwardlink is split into a fundamental and a supplemental channel. Thefundamental channel is continuously transmitted from all base stationsin the active set. The supplemental channel is transmitted from the samebase stations as the fundamental channel or a subset thereof. Theinvention described herein can be applied to the fundamental channel,the supplemental channel or both.

[0111]FIG. 9 is a spectrum diagram of a multi-carrier spread spectrumforward link and a single carrier wide-band spread spectrum link.Although not shown completely to scale, for the multi-carrier approach,the spreading bandwidth for each carrier is shown as 1.25 MHz, and forthe single carrier wide-band approach the spreading bandwidth is 3.6864MHz. The multi-carrier approach has various advantages includingallowing each carrier to be transmitted from a differently configuredantenna, which in turn provides a unique fading pattern for each carrierdecreasing the likelihood of all three carriers fading simultaneously,and therefore of communications from being disrupted.

[0112]FIG. 10 is a block diagram of a multi-carrier transmit systemconfigured in accordance with one embodiment of the invention. Inputdata is convolutionally encoded and punctured by conventional encoder100, and the encoded symbols are repeated by symbol repeater 102 to addadditional redundancy. Block interleaver 104 block interleaves therepeated symbols in 20 ms time intervals and the interleaved symbols arescrambled via XOR 106 with a decimated long code generated by long codegenerator 110 and decimator 108 in response to a user long code mask.The scrambled symbols are demultiplexed by demux 112 into three symbolstreams that are each transmitted over a respective carrier signal.

[0113] For each carrier signal, the respective symbol streams are QPSKmapped by QPSK mappers 114. The QPSK symbols are each modulated with thesame Walsh channel code by Walsh code modulators 116 and the resultingWalsh chips are further modulated by with an in-phase spreading codePN_(I) and a quadrature-phase spreading code PN_(Q) by spreaders 118.PN_(I) and PN_(Q) are preferably the same for each carrier. Theresulting spread symbols are then each upconverted to a unique carrierfrequency, preferably as shown in FIG. 9, and transmitted. FIG. 10 showsmodulation by the same Walsh channel code for each carrier; however, theWalsh channel code may be different.

[0114]FIG. 11, is a block diagram of a portion of a receive systememployed by a mobile unit to process a multi-carrier signal whenconfigured in accordance with one embodiment of the invention.Downconverted RF energy is bandpass filtered to 5 MHz by bandpass filter200 and sampled by A/D 202 at a rate of 8×1.2288 MHz. Within filter bank204, two 1.25 MHz portions of the samples are further downconverteddigitally to baseband by a 1.2 MHz numerically controlled oscillator(NCO), or optionally by a 1.25 MHz NCO and a 2.5 Mhz NCO, and the threesets of samples are lowpass filtered to a 1.25 MHz bandwidth. Thislowpass filter can be the receiver's matched filter or a part thereof.The resulting sets of lowpass filtered data is passed to rake receiver210, which demodulates and combines the various multipath instances ofthe transmitted signal. The resulting combined soft decision data ispassed on to a deinterleaver for deinterleaving and then decoding.

[0115] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A method for adjusting forward traffic channelpower allocation in a communications system, comprising the steps of:measuring at a mobile station respective signal qualities of pilotsrespectively transmitted by a plurality of base stations in an activeset of said mobile station; comparing said respective signal qualitiesof said pilots to a standard; reporting a message to a system controllerindicating which of said pilots at said mobile station equal or surpasssaid standard; and adjusting said forward traffic channel powerallocation based on said message.